With the increased incorporation of electronic components and electrical systems in modern motor vehicles, there is a greater need for components and systems having circuits with high-current and better thermal management capabilities. Circuits with such capabilities are necessary to satisfy the high-power applications and requirements which are common in such modern motor vehicles. In addition to automotive applications, such circuits may lead to improved thermal management in electronic devices ranging from SCR's to high power MOSFET's and microprocessors. In general, for a circuit to meet such criteria, the metal conducting path must be sufficiently thick to minimize heat generation and to conduct or spread the heat or thermal energy to assist in circuit cooling.
One prior art solution to fabricating thick circuits with high-current and better thermal management capabilities has been to utilize ultra-thick film technology in which a circuit line of silver is screen printed in single or multiple layers, depending on the required thickness of the circuit. For examples, see U.S. Pat. No. 5,395,679 issued to Myers et al on Mar. 7, 1995, and see also U.S. Pat. No. 5,527,627 issued to Lautzenhiser et al on Jun. 18, 1996. However, in attempting to form a circuit line of a particular thickness by overlay printing of multiple layers of silver, the first layer is limited to a practical thickness of only about 0.15 millimeters (6 mils). Additional layers are limited to thicknesses of only about 0.075 millimeters (3 mils). In addition to having to print multiple layers on top of each other to obtain a circuit line with sufficient thickness, it is also necessary to fire each deposited layer before proceeding to deposit the next layer. Thus, a first drawback to this method is that it requires multiple processing steps and additional time to form a sufficiently thick multi-layer circuit line wherein each layer must be fired before the next layer can be applied. In addition, a second drawback to this method is that, when dealing with surface mount technologies (SMT), there are significant problems associated with soldering a pre-fabricated electronic component to a silver circuit line. This includes silver leaching during the solder reflow process, and the undesired growth of brittle tin-silver intermetallic compounds.
Another approach is to utilize a process for the metallization of alumina called “direct bond copper.” See, for example, U.S. Pat. No. 3,993,411 issued to Babcock et al on Nov. 23, 1976. In this process, a thin copper foil is directly bonded to an alumina substrate under a very stringently controlled environment. The desired circuit line, which is to be formed from the copper foil, is covered and protected with a chemical-resist mask, and the remaining copper foil which is not protected by the mask is thereafter etched away. In this way, the remaining un-etched copper foil defines the circuit line. However, with this approach, the-thickness of the copper circuit line is limited to about 0.5 millimeters (20 mils) because a thicker copper circuit line would begin to exhibit poorer adhesion to the alumina substrate. In addition, a thicker copper foil is more expensive to process because of the additional time required to etch the unprotected thicker copper foil.
Although both of the above-mentioned methods involve standard processes, neither one of them provides a low-cost way to fabricate sufficiently thick circuit lines having high-current and good thermal management capabilities. Thus, there remains a present need in the art for the low-cost fabrication of thick films of copper, preferably several millimeters thick, having high-current and good thermal management capabilities. An example of a high current application is one requiring electrical current on the order of 10 to 200 amperes.